Inversion centers, crystalline polymers

He concluded that this remarkable difference in physical properties resulted from identical configuration of all the asymmetric centers in the crystalline polymer. The liquid polymer from racemic monomer, on the other hand, was evidently a stereorandom atactic polymer (4). Both polymers were later shown to be formed almost exclusively by head-to-tail polymerization. Vandenberg, Price, and others later showed that addition of each epoxide unit to the polymer chain occurs with inversion of configuration at the carbon atom where ring opening occurs. The asymmetry in this case is not disturbed, however, because the bond between the oxygen and the asymmetric carbon is never broken. 

In the infrared spectrum of polyethylene (Fig. 4.1-2A) this band is split into a doublet at 720 and 731 cm This factor group splitting (Fig. 2.6-1 and Sec. 2.7.6.4) is a result of the interaction between the molecules in crystalline lattice areas. It may be used to investigate the crystallinity of polymers (Drushel and Iddings, 1963 Luongo, 1964). Polyethylene has a unit cell of the factor group Dih (compare Secs. 2.7.5 and 2.7.6.3) which contains a -CH2-CH2- section of two neighboring chains. Each of these sections has a center of inversion in the middle of the C-C bond (Fig. 4.1-3). Therefore the rule of mutual exclusion (Sec. 2.7.3.4) becomes effective The vibrations of the C-C bonds cannot be infrared active and further there are no coincidences of vibrational frequencies in the infrared and Raman spectrum of linear polyethylene. 

C.J. Hawker and J.M.J. Fechet, Preparation of polymers with controlled molecular architecture. A new covergent approach to dendritic macromolecules, J. Am. Chem. Soc., 1990, 112, 7638 F. Zeng and S.C. Zimmerman, Dendrimers in supramolecular chemistry from molecular recognition to self-assembly, Chem. Rev., 1997, 97, 1681 D.J.P. Yeardley, G. Ungar, V. Percec, M.N. Holerca and G. Johnsson, Spherical supramolecular minidendrimers self-organized in an inverse micellar -like thermotropic body-centered cubic liquid crystalline phase, J. Am. Chem. Soc., 2000, 122, 1684. 

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